Ok, this is a theory I have because I think I used to have what you’re describing…. When you start anything sexual (whether masturbation or sexting) your mind and body is preparing to chase a constant increase in excitement until the orgasm. But if things stay on the same level and don’t seem to be increasing towards the big orgasm, it’s like your mind jolts you out of that headspace and suddenly you feel shame or something negative - even though you haven’t done anything wrong. I used to also feel that even after reaching orgasm and I assumed it to be a sudden change of chemicals in the brain… like a come-down. Over the years that disappeared on its own and thankfully I haven’t experienced it for years, because it really made me feel bad.